Emissions of heavy metals such as Hg, Ni, Cr, Cd, Co, Pb, V, Se, Be, As, Zn, etc. have become environmental issues of increasing importance because of the dangers to human health. Mercury is a trace element of particular concern, because during coal and municipal solid waste combustion, most of the mercury present in coal and municipal solid waste is transferred into the vapor phase due to its high volatility. Currently available pollution abatement technologies are not capable of effectively controlling gas phase mercury emissions at high temperatures particularly from flue gas emissions in the utility industry. Once discharged to the atmosphere, mercury persists in the environment and creates long-term contamination problems. Furthermore, well documented food chain transport and bioaccumulation of mercury require strict control of mercury emissions from coal-fired power plant and other sources.
Present mercury emission control technologies such as adsorption using various absorbents, direct carbon injection, flue gas desulfurization technologies (FGD), wet scrubbers, wet filtration, etc. are still limited to research stages. None of these technologies have been shown to completely remove mercury, in particular elemental mercury, from gas streams, particularly above ambient temperature.
Among these technologies, adsorption on sulfur-impregnated carbon has shown some promise with removal of 50-90% mercury in flue gases depending on reaction conditions. Sulfur is introduced into activated carbon by impregnating with different forms of sulfur such as elemental sulfur, carbon disulfide, hydrogen sulfide, or sulfur dioxide. Because sulfur is deposited on the surface of the activated carbon, there are some disadvantages with either final products or the process such as 1) uniformity of the sulfur over the surface is questionable due to the heterogeneity of the carbon surface; 2) the amount of sulfur in the carbon is limited; 3) pore opening is significantly decreased after formation of the carbon-sulfur complex on the surface (pore entrance) and consequently the surface area is reduced; 4) chemical interaction between sulfur and carbon may be weak 5) introduction of other additives is restricted by competition with sulfur for surface active sites in carbon; and 6) physical shape of final carbon is limited to granules or powder.
A need exists, therefore, for a more homogeneous and efficient mercury removal catalyst. The present invention provides such a catalyst.